Yamaha pm1000 120hz hum

[Ifudontlikebees]

Hey everyone! I’ve posted a few times here about a pm1000 that I’ve got but having some more issues with it. I got the power supply working and recapped it, recapped all of the the input modules, and added unbalanced direct outs via db25 connectors from C37. Everything is sounding great with the exception of a 120hz hum. It was there before and I thought that recapping everything would help but I didn’t. The hum continues to happen when the master modules are pulled. Anyone have any idea what could cause that or where I could start troubleshooting?

 

[Brian Roth]

Check the 44 Volt power supply rail which feeds all the audio stages. The voltage should be very close to 44 VDC. Hopefully you have a scope so you can use AC coupling and look at the noise floor of that rail.

Bri

 

[Ifudontlikebees]

The dc connector is putting out 55vdc, and the rail I believe is getting close to 49vdc. No scope unfortunately

 

[JohnRoberts]

Hey everyone! I’ve posted a few times here about a pm1000 that I’ve got but having some more issues with it. I got the power supply working and recapped it, recapped all of the the input modules, and added unbalanced direct outs via db25 connectors from C37. Everything is sounding great with the exception of a 120hz hum. It was there before and I thought that recapping everything would help but I didn’t. The hum continues to happen when the master modules are pulled. Anyone have any idea what could cause that or where I could start troubleshooting?

Troubleshooting noise like hum inside a console requires isolating at what stage of the signal path does the hum get introduced?

Recapping with the possible exception of unregulated PS reservoir caps generally has no impact on hum. In power supplies ripple voltage can contribute to hum indirectly.

How do the controls between the input and output affect the hum? Is there hum in the insert/direct out.

JR

 

[Brian Roth]

The dc connector is putting out 55vdc, and the rail I believe is getting close to 49vdc. No scope unfortunately

55 dcv measured where....same with 49v.

Bri

 

[user 130681]

The emitter of Q3 must have 44 volts on it. No more because if there is more, it is not filtering properly.

 

[Ifudontlikebees]

Thanks for the replies everyone! The top of the dc connector measures 50v where it’s supposed to put out 44, and I’m getting roughly 54 volts on pin 15 of the input modules which powers the modules.

 

[Ifudontlikebees]

I can bring down the trimmer to 44v but it still produces higher voltage on the modules

 

[Brian Roth]

Thanks for the replies everyone! The top of the dc connector measures 50v where it’s supposed to put out 44, and I’m getting roughly 54 volts on pin 15 of the input modules which powers the modules.

You are somehow making a mistake with your measurements. Pin 15 (and 1) on all modules connects to pin 1 on the DC power connector through a 10 Ohm resistor.

It is impossible for pin 15/1 to be HIGHER than what is provided from the output of the regulator.

Bri

 

[Ifudontlikebees]

Weird! I double checked pin 15 and it’s measuring as 54 volts.

 

[radardoug]

And its supposed to be 44 volts? Possibly your regulator is shorted or faulty.

 

[Ifudontlikebees]

The zener diode ladder was in fact blown and I replaced it which caused the console to come back to life. That was awhile ago though. What else would I have to check to see if the regulator circuit is blown? I don’t know much about regulator circuits.